This invention relates to a denitration catalyst for reducing nitrogen oxides in an exhaust gas with a reducing gas such as ammonia and thus render them non-toxic. More specifically, it relates to a catalyst which even when the exhaust gas contains an arsenic compound, can continue to have reducing activity on nitrogen oxides over a long period of time without being readily poisoned by the arsenic compound.
Methods have been known to reduce nitrogen oxides (to be sometimes referred to as NO.sub.x) contained in exhaust gases from various fixed generation sources, particularly exhaust gases from various combustion furnaces such as a boiler, by contacting them with reducing gases such as ammonia and thus render them non-toxic.
Various catalysts heretofore known for use in such methods have problems still desired to be solved in regard to activity, strength and cost, and also to durability to sulfur compounds, arsenic compounds, etc. contained in the exhaust gases. Particularly, when the exhaust gases contain arsenic compounds, most of the catalysts are poisoned by the arsenic compounds and rapidly lose activity during use.
Arsenic compounds have long been known to be activity poisoning substances against catalysts used in various reactions. A typical poisoning action of arsenic compounds is seen in hydrogenation reaction with a platinum or palladium catalyst in the liquid phase or a reaction of synthesizing anhydrous sulfuric acid from SO.sub.2 using a vanadium pentoxide catalyst.
This catalyst poisoning action of arsenic compounds is similarly observed in the reaction of reducing NO.sub.x in exhaust gases to render them non-toxic. Many denitration catalysts rapidly lose activity in the presence of arsenic compounds in exhaust gases and become industrially useless.
Boiler exhaust gases from burning of coal, heavy oils, etc. as fuels and exhaust gases of a glass melting furnace frequently contain arsenic compounds. One example of a particularly high arsenic content is in a boiler exhaust gas resulting from burning of coal occurring in Europe.
Many of the catalysts previously proposed for use in reducing NO.sub.x in exhaust gases with reducing gases and thus rendering them non-toxic have the disadvantage of undergoing poisoning by arsenic compounds in the exhaust gases and rapidly losing NO.sub.x reducing activity.
The active lifetime of catalyst greatly affects the smooth operation of the apparatus and the economy of the denitration process, and it cannot be denied that the short catalyst life is very disadvantageous in industrial operations.